1. Field of the Invention
This invention relates to switching devices and, more particularly, to a switching device suitable for switching of transmission path.
2. Description of the Related Art
With the recent significant advancement in the field of information and communications technology, the frequency bands of signals that communication equipment is required to deal with has been developed to higher frequency band levels ranging from microwave band to millimeter wave band. Communication circuits dealing with high frequency bands ranging from microwave band to millimeter wave band usually employ a switch of the path switching type for transmission line control.
Generally, a typical switch of the path switching type comprises a combination of PIN diode switches making use of a semiconductor P/I/N junction or a combination of FET (field-effect transistor) switches making use of a FET switching function. For example, in a switching device of the path switching type composed of two FET switches connected to respective transmission paths as switching targets, one of the two FET switches is turned on (off) while the other FET switch is turned off (on). More specifically, the FET switches operate in a complementary manner, such that the one FET switch is turned on when the other switch is turned off, and the other FET is turned on when the one FET switch is turned off for switching of transmission path.
On the other hand, the demand for the miniaturization of high frequency circuits becomes increasingly stronger in comparison with other electronic circuits. Generally, the high frequency circuit is formed as an MMIC (Monolithic Microwave IC) in which a semiconductor device (e.g., a high frequency transistor), a matching circuit, a bias circuit et cetera are integrated with each other on a single semiconductor substrate. For the case of the MMIC, preferably the switch itself is composed of a semiconductor device. Consequently, commonly-used MMICs employ for example PIN diodes or FET switches. However, the process of fabricating a PIN junction is complicated in comparison with the process of fabricating a FET. Therefore, it is preferable to form a switching device only with FETs.
In the FET switch, the control voltage is applied to a gate electrode of a FET so that the channel conductivity changes to vary the source-drain conductivity. Thereby, according to the source-drain conductivity variation, the amount of transmission of the transmission signal between the source and the drain is varied. In other words, the FET switch is turned on when the channel layer of the FET is placed in the electrically conductive state, and the transmission signal is input to one of the drain and source terminals, passes through the channel, and is output from the other of the drain and source terminals. On the other hand, the FET switch is turned off when the channel layer is placed in the pinch-off state, and the source and the drain are electrically disconnected from each other. For the case of high frequency signals, generally HEMTs (High Electron Mobility Transistors) in which n-type channel layers are formed are employed.
In order that a plurality of FET switches composed of FETs of the same channel type can operate in a complementary manner (in other words, certain of the plural FET switches are turned on (off) while the remaining FET switches are turned off (on)), it is required that FETs of the certain FET switches and FETs of the remaining FET switches be given different control voltages so that each FET switch performs a switching operation. However, in the light of ease of control and circuitry simplification, it is desirable that a plurality of FETs operate in a complementary switching manner with a single control voltage.
Furthermore, for the case of frequently-used n-channel FETs of the depletion type, it is necessary to apply a negative potential (hereinafter referred to as “the negative voltage”) with respect to the source potential to the gate electrode so that the channel is placed in the pinch-off state. However, generally the source electrode is grounded, so that, when employing such a depletion type n-channel FET as a switching device, there must be provided a negative power supply for gate electrode control in addition to the provision of a positive power supply for drain biasing.
Additionally, for the case of high frequency signals, when a transmission path is switched, if a transmission line of the disconnected side remains in the open state, an impedance of the transmission line becomes discontinuous at the open point and, as a result, signal reflection occurs at the open point. This high frequency signal reflection makes the circuit characteristics worse, resulting in unstable circuit operations.
Furthermore, the FET has a channel resistance. Because of this, insertion of a switch composed of FETs results in transmission loss due to the FET channel resistance.
In addition to the above-mentioned technologies, there are other technologies about switching devices for use in transmission path switching (see Japanese Pat. No. 2848502, Japanese Pat. No. 3068065, Japanese Pat. Kokai Pub. No. (1992)33501, Japanese Pat. Kokai Pub. No. (2000)349502, Japanese Pat. Kokai Pub. No. (1990)90723, Japanese Pat. Kokai Pub. No. (1996)213891, Japanese Pat. Kokai Pub. No. (1991)145801, Japanese Pat. Kokai Pub. No. (1992)346513, Japanese Pat. Kokai Pub. No. (1994)85641, Japanese Pat. Kokai Pub. No. (1998)313266, Japanese Pat. Kokai Pub. No. (1998)335901, Japanese Pat. Kokai Pub. No. (1995)235802, Japanese Pat. Kokai Pub. No. (1994)132701, Japanese Pat. Kokai Pub. No. (2002)141794, Japanese Pat. Kokai Pub. No. (1996)288400, Japanese Pat. Kokai Pub. No. (1997)27736, and Japanese Pat. Kokai Pub. No. (1997)107203).